The 55-year-old was convicted in 2003 of murdering Lisha Gayle in a burglary that appeared to get out of hand.

Marcellus Williams, convicted of murder 21 years ago, has been executed in the Midwestern state of Missouri despite concerns about the integrity of the case.

The United States Supreme Court, the last court that could have overturned Williams’ death sentence, declined to intervene in the case on Tuesday.

The 55-year-old was executed by lethal injection at a prison in Bonne Terre shortly after 6 p.m. (23:00 GMT), according to The Innocence Project, whose lawyer worked with Williams. His death came a day after both Missouri Gov. Mike Parson and the state’s highest court rejected his final appeals to prevent his execution.

Williams was found guilty of the 1998 murder of Lisha Gayle, a 42-year-old former newspaper reporter who was stabbed 43 times during what appeared to be a botched burglary. He maintained his innocence.

Wesley Bell, whose office handled the original prosecution, had tried to block the execution over concerns about the original trial.

“Even for those who disagree with the death penalty, the irreversible punishment of execution should not be an option if there is even a shadow of doubt about a defendant’s guilt,” Bell said in a statement before the execution.

In court papers, Bell questioned the reliability of the two key witnesses, concluded that prosecutors had wrongly excluded black jurors based on race, and noted that new tests found no trace of Williams’ DNA on the murder weapon. Williams was African-American.

Later testing also revealed DNA on the knife from a prosecutor and a detective who worked on the case and handled the weapon without gloves.

The contamination of the knife led prosecutors and Williams’ attorneys to agree in August to commute the sentence to life in prison.

Gayle’s family also supported the deal, but Missouri Attorney General Andrew Bailey opposed it and the state Supreme Court blocked it at his request. A state judge upheld Williams’ murder conviction earlier this month, ruling that the lack of evidence on the knife was not enough to establish his innocence.

The Missouri Supreme Court affirmed that decision on Monday.

Governor Parson, a Republican, also denied Williams’ request for clemency that same day.

“No jury or court, including at the trial, appellate and Supreme Court levels, has ever found Mr. Williams’ claims of innocence valid,” he said in a statement. “Ultimately, his conviction and death sentence were upheld.”

Williams was among death row inmates in five states scheduled to be executed within a week, an unusually high number given the years of decline in the use of and support for the death penalty in the United States.

The first was carried out Friday in South Carolina. Texas was also scheduled to execute an inmate Tuesday night. Travis Mullis, 38, was convicted of bludgeoning his 3-month-old son, Alijah Mullis, to death in 2008.

The death penalty has been abolished in 23 US states, while six others – Arizona, California, Ohio, Oregon, Pennsylvania and Tennessee – have a moratorium.